This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-146630, filed May 18, 2000, the entire contents of which are incorporated herein by reference.
This invention relates to a method of manufacturing a composite piezoelectric substance adapted for use in an ultrasonic probe, etc., and also to a method of working a piezoelectric substance to be employed in the manufacture of a composite piezoelectric substance.
As shown in xe2x80x9cHand Book of Ultrasonic Diagnostic Equipments, Revised Edition; Nippon Electronic Mechanical Industries Association; Corona Publishing Co., Ltd., 1997, 1/20, p68-74xe2x80x9d, the ultrasonic probe is constructed such that it is mainly consisted of a piezoelectric element, an acoustic matching layer, an acoustic lens and a backing material, all of which are integrated with each other. The piezoelectric element is formed of a piezoelectric ceramic board with electrodes on the opposite surfaces thereof. The acoustic matching layer and the acoustic lens are formed on a side of the piezoelectric element where an ultrasonic wave is transmitted or received. The backing material is formed on the back side of the piezoelectric element.
This ultrasonic probe is operated as follows. A driving pulse of one hundred to several hundreds volts is transmitted from a pulser to the piezoelectric element. Due to this driving pulse, the piezoelectric element is suddenly deformed by a reverse piezoelectric effect. As a result of this deformation, the ultrasonic pulse is excited and emitted via the acoustic matching layer and the acoustic lens.
The ultrasonic pulse thus oscillated is then reflected by an object. The ultrasonic pulse thus reflected re-enters via the acoustic lens and the acoustic matching layer into the piezoelectric element, thus mechanically vibrating the element. This mechanical vibration of the element is converted by way of piezoelectric effect into an electric signal which is then transmitted to a monitoring device and reproduced as an image. The object which reflects the ultrasonic pulse is an interface between tissues of one""s body when the pulse is employed for a medical purpose. The object is a discontinuous portion in a measuring object, such as flaw, when the pulse is employed for a non-destructive examination.
Generally, a piezoelectric ceramic is employed for the manufacture of the piezoelectric element of the ultrasonic probe. In recent years, however, a composite piezoelectric transducer using a composite piezoelectric substance consisting of a piezoelectric rod (rod-shaped piezoelectric ceramics) and a resin has been actually utilized as an electromechanical energy transducer for the piezoelectric element.
As set forth in Japanese Patent Unexamined Publication S60-85699 (hereinafter, referred to as a prior art 1), one example of the conventional method of manufacturing the composite piezoelectric substance involves a step of dicing a block-shaped piezoelectric substance (piezoelectric block) to manufacture piezoelectric rods. This method is performed as follows. First of all, a piezoelectric block of, for example, lead zirconate titanate (PZT) is adhered to a substrate with an adhesive. Then, the piezoelectric block on the substrate is diced with a dicing device to obtain a plurality of piezoelectric rods. The groove portion formed as a result of this dicing is filled with a resin such as epoxy resin or urethane resin, and then, the resin is allowed to cure. Finally, the piezoelectric block diced in this manner is removed from the substrate to obtain a composite piezoelectric substance.
More specifically, above mentioned manufacturing method can be performed by two different procedures. One of the procedures involves the steps of completely cutting the piezoelectric block by way of dicing, filling the groove with a resin, curing the resin, and removing the piezoelectric block from the substrate to obtain the composite piezoelectric substance. The other procedure involves the steps of dicing incompletely the piezoelectric block, filling the groove portion with a resin, curing the resin, removing the piezoelectric block from the substrate, and grinding or slicing the piezoelectric block to obtain the composite piezoelectric substance.
Another example of manufacturing method of the composite piezoelectric substance is set forth in xe2x80x9cJpn. J. Appl. Phys.xe2x80x9d; Vol. 36(1997), pp. 6062-6064 (hereinafter, referred to as a prior art 2). This prior art discloses a method of manufacturing a composite piezoelectric transducer, wherein a deep X-ray lithography and a resin molding method are combined to obtain piezoelectric rods having a high aspect ratio, and then, the space formed between the piezoelectric rods is filled with a resin to obtain the composite piezoelectric transducer.
More specifically, first of all, a resist film having a thickness of 400 xcexcm and consisting of MMA (methyl methacrylate)/MAA (methacrylic acid) copolymer is deposited on a substrate. Then, a synchrotron radiation is irradiated via a mask onto the resist film, which is followed by the development of the resist film (deep etch X-ray lithography), thereby obtaining a resist structure having a plurality of apertures. Thereafter, a PZT (lead zirconate titanate)
slurry is poured into these apertures of the resist structure. The pouring of the PZT slurry, which is consisted of a PZT powder, a binder and water, is performed utilizing the resist structure as a resin mold.
Then, the PZT slurry is allowed to dry and cure at a room temperature to obtain a PZT green body. Then, only the resin mold is removed by means of an oxygen plasma, thus leaving the PZT green body. The PZT green body thus left is next subjected to a defatting treatment (the removal of the binder) at a temperature of 500xc2x0 C., and then to a main sintering treatment at a temperature of 1,200xc2x0 C. As a result of the sintering, a PZT rod array consisting of a plurality of PZT rods (piezoelectric columns) each having a dimension of 20 xcexcm in diameter and 140 xcexcm in height is obtained.
Then, the space between the PZT rods is filled with epoxy resin by means of vacuum impregnation, which is followed by the curing of the epoxy resin to obtain a composite piezoelectric substance. Thereafter, the top and bottom surfaces of the rod array are polished until the opposite end faces of the PZT rods are exposed. Thereafter, gold electrodes are deposited by means of sputtering on the flattened top and bottom surfaces of the rod array. Thereafter, the rod array is subjected to a poling treatment by impressing a voltage onto the electrodes under the condition wherein the rod array is kept immersed in an oil bath. As a result, a small/thin type composite piezoelectric transducer constituted by a composite piezoelectric substance having a piezoelectric property can be obtained.
However, the aforementioned conventional methods of manufacturing the piezoelectric structure are accompanied with the following problems.
(1) In the method set forth in the prior art 1, if the diameter of the piezoelectric rod is made too small by way of the dicing, the piezoelectric rod may be easily be destroyed during dicing. Therefore, it has been very difficult to obtain a piezoelectric rods of a small diameter, for instance 50 xcexcm or less, so that it has been difficult to manufacture piezoelectric rods of a high aspect ratio. As a result, it has been difficult to manufacture a composite piezoelectric transducer exhibiting a very high oscillating frequency which is required for improving a resolution of a diagnostic apparatus. Further, the piezoelectric rods manufactured by dicing work can only have a configuration of a polygonal cross-section and walls extending straight along a longitudinal axis thereof. Therefore, a redundant oscillation mode in a direction perpendicular to the longitudinal axis can arise in the vicinity of the resonance frequency in the direction of the longitudinal axis of piezoelectric rod, thereby causing noises in the ultrasonic probe.
(2) In the method set forth in the prior art 2, falling-down of the piezoelectric rods during sintering makes it difficult to reduce the diameter of the rods to less than several tens micrometers. Therefore, it has been very difficult to increase the aspect ratio of the rods and therefore to obtain a composite piezoelectric oscillator exhibiting a very high oscillation frequency.
Moreover, the resin mold makes it impossible to utilize a hot press or a hot isostatic press (HIP). Therefore, it has been impossible to highly densify the piezoelectric substance in order to sufficiently utilize its excellent intrinsic properties.
Additionally, it is substantially impossible to apply the method in the prior art 2 to a piezoelectric single crystal.
Therefore, the object of the invention is to provide a method of working a piezoelectric substance to obtain a piezoelectric structure, which is possible to finely work the substance and is also applicable to a piezoelectric single crystal, and a method of manufacturing a composite piezoelectric substance using the piezoelectric structure.
Accordingly, this invention provides a method of working a piezoelectric substance, which comprises the steps of;
forming, on one surface of a piezoelectric block, an etching mask having an aperture which defines a boundary region between a first piezoelectric segment to be removed, and a second piezoelectric segment to be en left remained,
forming, on the opposite surface of the piezoelectric block, a sacrificial layer which corresponds to the first piezoelectric segment to be removed and the boundary region,
etching the piezoelectric block in the boundary region to reach the sacrificial layer, and
eliminating the sacrificial layer to remove the first piezoelectric segment.
This invention also provides a method of manufacturing a composite piezoelectric substance, which comprises the steps of;
forming, on one surface of a piezoelectric block, an etching mask having an aperture which defines a boundary region between a first piezoelectric segment to be removed, and a second piezoelectric segment to be left remained,
forming, on the opposite surface of the piezoelectric block, a sacrificial layer which corresponds to the first piezoelectric segment to be removed and the boundary region,
etching the piezoelectric block in the boundary region to reach the sacrificial layer, eliminating the sacrificial layer to remove the first piezoelectric segment,
filling at least partially a space formed by the removal of the first piezoelectric segment with an organic material, and
forming, on the one surface and the opposite surface of the piezoelectric block, electrodes for driving the second piezoelectric segment left remained.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.